This invention relates to a semiconductor switch and a temperature sensing circuit for such a switch.
U.S. Pat. No. 4,730,228, issued Mar. 8, 1988, describes a temperature sensing circuit in which the temperature of a power semiconductor device is sensed by a bipolar transistor. The bipolar transistor is arranged in series with the main conductive path of a depletion mode (normally on) insulated gate field effect transistor (IGFET or MOSFET). The bipolar transistor has an open base connection and is in thermal contact with the power semiconductor device. For example, the bipolar transistor may be provided in the same semiconductor body as the power semiconductor device within a recess in the power semiconductor device. The depletion mode IGFET may also be integrated in the same semiconductor body but at a location remote from the power semiconductor device. In operation of the circuit described in said patent, when the circuit is at room temperature an off-state or leakage current flows through the open-base bipolar transistor and the depletion-mode IGFET acts as a current source. As the temperature of the power semiconductor device, and thus of the bipolar transistor, increases, the current through the bipolar transistor increases until it reaches that demanded by the depletion-mode IGFET. The internal resistance of the IGFET then increases sharply, resulting in production of a control signal for switching off the power semiconductor device.
Although the circuit described in said patent enables the thermally responsive bipolar transistor to be placed in good thermal contact with the power semiconductor device, in practice significant modifications would have to be made to the mask set used to manufacture the power semiconductor device in order to enable inclusion of the bipolar transistor.